This research is designed to investigate the role played by drug receptor sites in regulating the proliferation and differentiation of the hemopoietic stem cell. The study seeks to uncover receptor sites that switch DNA synthesis on or off in hemopoietic cells; i.e. alter the cell-cycle state of the bone marrow stem cell. Any role played by drug receptors in the leukemogenic process or in the adverse effects on bone marrow of clinically useful drugs is being studied. Such manipulation of the cell cycle may be a means of altering the sensitivity of tissues to cycle-dependent anticancer agents or cycle-dependent drugs used in immunosuppressive treatment. Hence, attempts will be made to promote a more selective action of cytotoxic drugs on specific tissues.